chore(core) Create instructions module (#9422)

## Summary
We have a variety of things we refer to as instructions in the code
base: our current canonical terms are:
- base instructions (raw string)
- developer instructions (has a type in protocol)
- user instructions

We also have `instructions` floating around in various places. We should
standardize on the above, and start using types to prevent them from
ending up in the wrong place. There will be additional PRs, but I'm
going to keep these small so we can easily follow them!

## Testing
- [x] Tests pass, this is purely a file move

codex-rs/core/src/codex.rs

@@ -98,6 +98,7 @@ use crate::error::Result as CodexResult;
 use crate::exec::StreamOutput;
 use crate::exec_policy::ExecPolicyUpdateError;
 use crate::feedback_tags;
+use crate::instructions::UserInstructions;
 use crate::mcp::auth::compute_auth_statuses;
 use crate::mcp_connection_manager::McpConnectionManager;
 use crate::model_provider_info::CHAT_WIRE_API_DEPRECATION_SUMMARY;
@@ -155,7 +156,6 @@ use crate::tools::spec::ToolsConfig;
 use crate::tools::spec::ToolsConfigParams;
 use crate::turn_diff_tracker::TurnDiffTracker;
 use crate::unified_exec::UnifiedExecProcessManager;
-use crate::user_instructions::UserInstructions;
 use crate::user_notification::UserNotification;
 use crate::util::backoff;
 use codex_async_utils::OrCancelExt;